Group B streptococcal strains, representing the five major serotypes, were inoculated into infant rats by intranasal, oral, and intraperitoneal routes. Bacteremia regularly followed injection by the intraperitoneal route. Four strains (three of type III) isolated from human cerebrospinal fluid appeared more virulent for 5-day-old rats. Injection of fewer than 10 colony-forming units of one strain, a type III, led to bacteremia and death in 27% of animals. The cumulative bacteremia and mortality rate with this strain was 66% in animals given inocula of less than 10 to 10(3) colony-forming units. Bacteremia developed by 24 to 48 h with concentrations of greater than 10(5) colony-forming units per ml of blood, and death occurred soon afterward. Among bacteremic animals, positive cerebrospinal fluid cultures were found in 97%, and cerebrospinal fluid bacterial concentrations were equal to or exceeded bacterial counts in blood. The susceptibility of infant rats to infection with type Ia, Ic, or III strains was age dependent. Histopathological studies of the brain and meninges in 34 bacteremic animals with culture-positive cerebrospinal fluid revealed that 5- to 10-day-old animals had numerous bacteria distributed in a perivascular pattern but, with one exception, no leukocytic infiltration. In contrast, three of the 11- to 12-day-old and two 15-day-old animals had very thickened meninges infiltrated with polymorphonuclear leukocytes, macrophages, and bacteria.